Motor vehicles such as automobiles or the like have a drive force transmitting mechanism for transmitting a drive force produced by an internal combustion engine to axles. The drive force transmitting mechanism includes, for example, a Birfield constant velocity universal joint and a tripod constant velocity universal joint that are connected to each other by a drive shaft.
The tripod constant velocity universal joint has an outer ring member disposed in a differential, not shown, and the Birfield constant velocity universal joint has an outer ring member for transmitting a rotational drive force to wheels, not shown.
The drive shaft has a distal end coupled to the outer ring member of the Birfield constant velocity universal joint by a plurality of rolling balls.
FIG. 10 shows an overall schematic perspective view of an outer ring member 1 for a Birfield constant velocity universal joint. The outer ring member 1 is made of carbon steel and has a shank (shaft) 7 and a cup 8 formed integrally with each other.
The cup 8 has six ball rolling grooves 9a through 9f defined in an inner wall surface at circumferentially spaced angular intervals. The ball rolling grooves 9a through 9f serve to allow balls (not shown) to roll therein, and extend along the inner wall surface of the outer ring member 1 nearly to a terminal end of the cup 8. The shank 7 has a positioning central hole, not shown, defined in an end thereof.
The outer ring member 1 is manufactured by cold forging as follows: First, as shown in FIG. 9A, a cylindrical workpiece 11 which is slightly greater in diameter than the shank 7 is pretreated. Specifically, the workpiece 11, which is made of carbon steel, is spheroidized (annealed) to produce the cementite in the form of globules in the metal structure, and then a lubricating chemical coating is formed on its surface by a bonderizing process. In the general cold forging process, a coating of zinc phosphate is often used as the lubricating chemical coating.
Then, a first forging process (forward extrusion) is performed on the workpiece 11 with the lubricating chemical coating thereon by a first forging die, not shown. Specifically, an end of the workpiece 11 is pressed toward a cavity defined in the first forging die and having a diameter smaller than the workpiece 11. The other end of the workpiece 11 is thus pressed into the cavity, producing a first form 13 having a reduced-diameter portion 12 tapered toward the other end and a shank 7, as shown in FIG. 9B.
Then, a second forging process (upsetting) is performed on the first form 13. Specifically, using a second forging die, not shown, only a large-diameter portion 14 of the first form 13 is progressively compressed to increase its diameter, producing a second form 15, as shown in FIG. 9C.
The second form 15 is then subjected to a low-temperature annealing process for removing stresses, a shot blasting process for removing oxide scales produced by the low-temperature annealing process, and a bonderizing process for forming a lubricating chemical coating made of zinc phosphate or the like on the outer surface of the second form 15.
After the second form 15 is thus treated, the second form 15 is placed in the cavity of a third forging die, not shown, and is subjected to a third forging process (backward extrusion) to extend the large-diameter portion 14 which has increased in diameter, form ball rolling grooves 17a through 17f in the large-diameter portion 14, and form a cup 8.
Specifically, a punch, not shown, having protrusions for forming the ball rolling grooves 17a through 17f is held against the center of an end face of the cup 8, and the distal end of the shank 7 is pressed to displace the second form 15 toward the punch. The second form 15 with the large-diameter portion 14 surrounded by the inner wall surface of the cavity is compressed by the punch, thereby extending the large-diameter portion 14 and forming ball rolling grooves 17a through 17f that are complementary in shape to the protrusions of the punch in the large-diameter portion 14. As a result, a third form 18 shown in FIG. 9D is produced.
Then, a low-temperature annealing process is performed on the third form 18 to soften the third form 18, and thereafter a lubricating chemical coating is formed thereon again by the shot blasting process and the bonderizing process. These processes are effective to prevent the inner surface of the cup 8 from cracking under tensile stresses at the time a next ironing process is performed.
Before the final ironing process is performed, a mark removing process is carried out to remove an annular ridge 19a as a mark formed on the edge of the inner surface of the cup 8 of the third form 18 and integrally projecting radially inwardly.
Specifically, in the shot blasting process performed on the third form 18, a projected mark is formed in the mouth of the inner wall surface in the opening of the cup 8 by steel balls ejected at a high speed and stirred. Such a mark is also formed when the form treated by the various forging processes falls into a storage container and hits other forms. The mark comprises the annular ridge 19a formed on the edge of the inner surface of the cup 8 and slightly projecting radially inwardly (see FIG. 11). If the projecting annular ridge 19a is not removed, then it will be difficult for an ironing punch, not shown, to be smoothly inserted into the cup 8 in the next process.
According to the mark removing process, the third form 18 is set on a mark removing machine (not shown) having a correcting punch, and the annular ridge 19a as the mark formed on the edge of the inner surface of the cup 8 is removed by the correcting punch.
Finally, an ironing process (final sizing process) for finishing the form to a final product shape, i.e., a fourth forging process, is performed by a fourth forging die, not shown, producing an outer ring member 1 for a Birfield constant velocity universal joint (see FIG. 9E).
After all the forging processes for forming the outer ring member 1 are finished, a cutting process such as lathing or the like is performed on the end face of the mouth of the cup 8 to deburr the cup 8, thereby producing an outer ring member 1 (see FIG. 10) for a Birfield constant velocity universal joint as a completed product.
Constant velocity universal joints and methods of manufacturing them according to the background art are disclosed in the following Patent Documents:
Japanese Laid-Open Patent Publication No. 11-236925 discloses a manufacturing method for forming, by plastic working, a beveled portion fully circumferentially on an inner circumferential surface of the mouth in the opening of the cup, without the need for a beveling process performed as a cutting process after the final ironing process.
Japanese Laid-Open Patent Publication No. 02-290640 discloses that a steel material which is of good formability and has a component ratio suitable for induction hardening is devised, and the steel material is upset at a possible forming rate.
Japanese Laid-Open Patent Publication No. 11-179477 discloses an ironing apparatus which is capable of reliably holding a shank and a cup in accurate coaxial alignment with each other and effectively keeping the inner surface of the cup accurate.
Japanese Laid-Open Patent Publication No. 11-182568 discloses a constant velocity universal joint having beveled portions produced only by plastic working on ridges provided at the boundaries between the inner circumferential surface of a cup and ball grooves.
Japanese Laid-Open Patent Publication No. 2003-083358 discloses a manufacturing method for ironing a form continuously before the form is work-hardened, without the need for a low-temperature annealing process and a lubricating chemical coating process on the form.
The above-mentioned forging method for producing a Birfield constant velocity universal joint according to the background art has a problem concerned with a mark removing process for removing a mark produced by shot blasting, etc. Since the diameter of a cup varies from automobile type to automobile type, the correcting punch of the mark removing machine has to be replaced depending on the automobile type involved. There are many replacement parts and a lot of labor is required for replacing them. As a result, the production efficiency is lowered, the manufacturing process is complicated, and the manufacturing cost is increased.